Equine Interleukin-15 (IL-15) is a cytokine that plays an important role in regulating innate and adaptive immune responses in horses (Equus caballus). IL-15 is produced by a variety of cell types, including monocytes, macrophages, dendritic cells, and epithelial cells, in response to infection or inflammatory stimuli. Equine IL-15 signals through a receptor complex composed of IL-15Rα together with the shared IL-2/15Rβ (CD122) and common γ chain (γc), activating intracellular signaling pathways such as JAK–STAT that promote lymphocyte proliferation, survival, and activation. In horses, IL-15 is particularly important for the development, maintenance, and activation of natural killer (NK) cells and memory CD8⁺ T cells, supporting cytotoxic immune responses against infected or abnormal cells. It also contributes to regulation of mucosal and inflammatory immune responses in tissues such as the respiratory and gastrointestinal tracts. In cell culture, recombinant equine IL-15 is commonly used to stimulate proliferation and survival of NK cells and cytotoxic T lymphocytes, maintain memory CD8⁺ T cell populations, and enhance cytotoxic activity in in vitro immune assays. Equine IL-15 is also studied in veterinary and translational research, including investigations of immune responses to equine infectious diseases such as equine herpesvirus and Rhodococcus equi, as well as studies of vaccine responses and immune regulation relevant to both equine health and comparative immunology.